Optical strain gages, and in particular an optical strain gage useful in a pressure transducer, are known in the art. U.S. Pat. No. 5,101,664, Optical Pressure Transducer, issued on Apr. 7, 1992 to Hockaday, et al shows a micromachine silicon pressure transducer which employs a single vibrating bridge and pressure responsive diaphragm formed from a single silicone wafer. The initial wafer is micromachined by a combination of etching and laser techniques so as to result in a single strand of silicon supported at each end by blocks or other supports also cut from the initial silicon wafer.
By forming the diaphragm, supports, and bridge from a single slab of homogeneous material, a pressure transducer according to Hockaday, et al is virtually unstrained by changes in temperature due to the uniformity of the thermal expansivity of the integral structure.
The degree of strain experienced by the bridge in the Hockaday configuration is determined by exciting the bridge structure by the use of a beam of light which is pulsed in intensity at a frequency equal to the nominal natural frequency of the bridge plus any strain-induced change in frequency. Optical devices, discussed in more detail in the Hockaday patent, monitor the vibration frequency of the bridge through the use of reflected light analyzed by an interferometer, and varies the pulse frequency of the driving light beam until the resident frequency of the bridge has been determined.
The amount of strain in the bridge is a function of the modulus of elasticity, geometry, and longitudinal stress present in the bridge, which in turn may be easily converted to strain at the surface of the diaphragm. The prior art device thus provides an effective, simple transducer for making temperature independent strain measurements, but requiring a relatively complicated measuring system able to interpret the measured frequency of the bridge and vary the driving frequency of the pulsed beam in order to seek out the current resonant bridge frequency.